Olefins having 3 or more carbon atoms can be polymerised to produce a polymer with an isotactic stereochemical configuration. For example, in the polymerisation of propylene to form polypropylene, the isotactic structure is typically described as having methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer. This can be described using the Fischer projection formula as follows; ##STR1##
Another way of describing the structure is through the use of NMR spectroscopy. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm with each "m" representing a "meso" diad or successive methyl groups on the same side in the plane.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is described as follows: ##STR2##
In NMR nomenclature, a syndiotactic pentad is described as . . . rrrr . . . in which "r" represents a "racemic" diad with successive methyl groups on alternate sides of the plane.
In contrast to isotactic and syndiotactic polymers, an atactic polymer exhibits no regular order of repeating unit. Unlike syndiotactic or isotactic polymers, an atactic polymer is not crystalline and forms essentially a waxy product.
The production of syndiotactic/isotactic block polypropylene is known from EP-A-0747406. In these known block polypropylenes there is a long isotactic block and few short syndiotactic blocks. A generically defined metallocene compound is disclosed as a catalyst component in the production of these block polypropylenes. The metallocene compound has a cyclopentadienyl ring which is monosubstituted, as exemplified by isopropylidene (3-trimethylsilylcyclopentadienyl-9-fluorenyl) zirconium dichloride and diphenylmethylidene(3-trimethylsilyl cyclopentadienyl-9-fluorenyl) zirconium dichloride.
The production of syndiotactic/atactic block polyolef ins is known from EP-A-0816475. In these known block polypropylenes there are alternating blocks of long syndiotactic and short atactic sequences. A generically defined metallocene compound is disclosed as a catalyst component in the production of these block polyolefins. The metallocene compound requires a hetero-atom ligand coordinated to the metal of the metallocene, as exemplified by 2,7-bis- tert-butyl-fluorenyl-9-dimethylsilyl-tert-butyl-amido titanium dichloride.
In EP-A-0423101 and EP-A-0742227 a metallocene compound is described which has a lack of bi-lateral symmetry, one example of which is isopropylidene (3-methyl cyclopentadienyl-1-fluorenyl)zirconium dichloride. This compound was used in the production of hemiisotactic polypropylene. The structure of hemiisotactic polymers may be represented in a Fischer projection as follows: ##STR3##
The monomeric unit of the polymer is of the following structure: ##STR4##
wherein R.sub.s is a hydrocarbyl group or nonhydrocarbyl group.
The structure of the polymer is characterised by R.sub.s groups attached to every other asymmetric carbon atom being on the same side of the principal polymer chain as represented in a Fischer projection and R.sub.s groups attached to the remaining asymetric carbon atoms being either on the same side or the opposite side of the principal polymer chain. Since only every other one conforms to the isotactic structure, it is "hemi". The material is a noncrystalline polymer.